1. Field of the Invention
The present invention relates to electrophysiology, and more particularly to an ECG monitoring and analyzing system for providing Frank X, Y and Z lead signals representative of electrical activity of a human heart using a reduced set of derived chest leads.
2. Brief Description of the Prior Art
Over the last sixty years, a variety of diagnostic procedures have been developed for sensing and analyzing the electrical activity of the human heart. These include: (a) electrocardiography, (b) vectorcardiography and (c) polarcardiography, all of which depend upon related apparatus used to produce records derived from voltages produced by the heart which are detected by electrodes placed on the surface of the subject""s body.
The records so produced are graphical in character and require interpretation and analysis to relate the resulting information to the heart condition of the patient or other subject. Historically, such records have been produced directly as visible graphic recordings from wired connections extending from the subject to the recording device. With advances in computer technology, it has become possible to produce such records in the form of digitally stored information for later replication of retrieval and analysis. Likewise, with advances in communication technology, not only has wireless sensing become possible, but also remote replication, retrieval and analysis of the acquired signals.
(a) Electrocardiography
The production of a conventional 12-lead electrocardiogram (ECG) involves the placement of 10 lead electrodes (one of which is a ground or reference electrode) at selected points on the surface of a subject""s body. Each electrode acts in combination with one or more other electrodes to detect voltages produced by depolarization and repolarization of individual heart muscle cells. The detected voltages are combined and processed to produce 12 tracings of time varying voltages. The tracings so produced are as follows:
where, in the standard, most widely used system for making short term electrocardiographic recordings of supine subjects, the potentials indicated above, and their associated electrode positions, are:
vL potential of an electrode on the left arm;
vR potential of an electrode on the right arm;
vF potential of an electrode on the left leg;
v1 potential of an electrode on the front chest, right of sternum in the 4th rib interspace;
v2 potential of an electrode on the front chest, left of sternum in the 4th rib interspace;
v4 potential of an electrode at the left mid-clavicular line in the 5th rib interspace;
v3 potential of an electrode midway between the v2 and v4 electrodes;
v6 potential of an electrode at the left mid-axillary line in the 5th rib interspace;
v5 potential of an electrode midway between the v4 and v6 electrodes;
vG (not indicated above) is a ground or reference potential with respect to which potentials vL, vR, vF, and v1 through v6 are measured. Typically, though not necessarily, the ground or reference electrode is positioned on the right leg.
Correct interpretation of an ECG requires a great deal of experience since it involves familiarity with a wide range of patterns in the tracings of the various leads. Any ECG which uses an unconventional system of leads necessarily detracts from the body of experience that has been developed, in the interpretations of conventional ECGs, and may therefore be considered generally undesirable. The recorded signals would be understandable only by a relative few who were familiar with the unconventional system.
Nevertheless, other lead systems have evolved from improvements in instrumentation that have permitted extension of electrocardiography to ambulatory, and even vigorously exercising subjectsxe2x80x94and to recordings made over hours, or even days. For example, in stress testing the electrodes are moved from the arms to the torso, although the same number of electrodes (10) are used. The tracings I, II, III, aVR, aVL and aVF are altered by this modification.
(b) Vectorcardiography
The pattern of potential differences on a body surface resulting from electrical activity of the heart can be mathematically approximated by replacing the heart with a dipole equivalent cardiac generator. The magnitude and orientation of this dipole are represented by the heart vector which is continually changing throughout the cycle of the heart beat. The XYZ coordinates of the heart give rise to time varying x, y and z signals, which may be written out as x, y and z tracings. Orthogonal leads to give these tracings were developed by Ernest Frank (see An Accurate, Clinically Practical System For Spatial Vectorcardiography, Circulation 13: 737, May 1956). Frank experimentally determined the image surface for one individual, and from this proposed a system using seven electrodes on the body, plus a grounding electrode. The conventional letter designations for such electrodes, and their respective positions were:
E at the front midline;
M at the back midline;
I at the right mid-axillary line;
A at the left mid-axillary line;
C at a 45.degree. angle between the front midline and the left mid-axillary line;
F on the left leg; and
H on the back of the neck.
The first five electrodes (E, M, I, A and C) were all located at the same transverse levelxe2x80x94approximately at the fourth of the fifth rib interspace. A linear combining network of resistors attached to these electrodes gave suitably scaled x, y and z voltage signals as outputs.
Unfortunately, x, y and z tracings are not as easy to interpret as 12 lead ECGs. However, Frank intended his system for a different purpose: vectorcardiography.
Although it has long formed a basis for teaching electrocardiography, vectorcardiography has never become widely used. The technique was demanding and the system of electrode placement was different from that required for conventional ECG""s. Extra work was required, and it would still be necessary to record a 12-lead ECG separately with a different placement of electrodes.
(c) Polarcardiography
An alternative representation of the heart vector, known as polarcardiography, has been exploited since the early 1960""s (see G. E. Dower, Polarcardiography, Springfield, Ill., Thomas, 1971). It has certain inherent advantages in defining abnormalities, and forms the basis of a successful program for automated analysis. Based on the x, y and z signals, polarcardiography employs the Frank lead system. In order to render it competitive with the established 12-lead ECG, the lead vector concept has been employed to derive a resistor network that would linearly transform the x, y and z signals into analogs of the 12-lead ECG signals called herein xe2x80x9cderived 12-lead signalsxe2x80x9d (see G. E Dower, A Lead Synthesizer for the Frank Lead System to Simulate the Standard 12-Lead Electrocardiogram, J. Electrocardiol 1: 101, 1968, G. E. Dower, H. B. Machado, J. A. Osborne, On Deriving the Electrocardiogram From Vectorcardiographic Leads, Clin Cardiol 3: 97, 1980; and G. E. Dower, The ECGD: A Derivation of the ECG from VCG leads (ecitorial), J. Electrocardiol 17: 189,1984). The derived 12-lead ECG is commonly referred to as the ECGD. Because the ECGD can be acceptable to an interpreting physician, it is not necessary for the technician to apply all the electrodes required for a conventional ECG. Further, associated computer facilities can make vectorcardiograms and other useful displays available from the x, y and z recordings. Nevertheless, the number of electrodes called for by the Frank lead system are required. In addition, the effort required by the technician recording the x, y and z signals is about the same as for a conventional ECG.
FIG. 1 illustrates a prior art patient monitoring system, such as manufactured and sold by Siemens Medical Systems, Inc. of Iselin, N.J., using the SC7000 Bedside Monitor 1, an Infinity Communication Network 2, and a MultiView Workstation (MVWS 3). As shown therein, limb lead electrodes RA, LA, RL, and LL are placed on a patient in the standard limb electrode positions. Chest electrodes V1, V2, V3, V4, V5, and V6 are placed on the patient in Frank electrode positions I, E, C, A, M, and H respectively. The contribution of the Frank electrode F is computed algebraically from the formula F=((2xc3x97lead II)xe2x88x92(lead I))/3.
The following linear equations represent the SMS-Prime lead to X, Y, Z transformation processing step 5, carried out in Bedside Monitor 1:
X=0.610*V4+0.171*V3xe2x88x920.781*V1xe2x80x83xe2x80x83(EQ 1)
Y=0.437*IIxe2x88x920.218*I+0.345*V5xe2x88x921.000*V6xe2x80x83xe2x80x83(EQ 2)
Z=0.133*V4+0.736*V5xe2x88x920.264*V1xe2x88x920.374*V2xe2x88x920.231*V3xe2x80x83xe2x80x83(EQ 3)
In FIG. 2, TABLE 1 is a matrix representation of the above equations.
The X, Y, and Z leads computed using the SMS Prime Lead to X, Y, Z Transformation are transformed in Monitor 1 using a reduced Dower Transformation processing step 6 outlined in TABLE 2 of FIG. 2. These two linear transformations combine to generate a set of derived leads in the bedside monitor 1 that are made available on the Communication Network 2. The set of derived leads available on Network 2 consists of derived(d) leads dI, dII, dIII, dV1, dV2, dV3, dV4, dV5, and dV6. An algebraic formula is used to derive the augmented leads locally on Monitor 1 in processing step 7 as shown below.
aVR=xe2x88x920.5(I+II)
aVR=Ixe2x88x920.5 (II)
aVF=IIxe2x88x920.5(I)
Given that L is a 9xc3x971 lead array representing lead values at a particular instant,   L  =      [                                        V            ⁢                          xe2x80x83                        ⁢            1                                                            V            ⁢                          xe2x80x83                        ⁢            2                                                            V            ⁢                          xe2x80x83                        ⁢            3                                                            V            ⁢                          xe2x80x83                        ⁢            4                                                            V            ⁢                          xe2x80x83                        ⁢            5                                                            V            ⁢                          xe2x80x83                        ⁢            6                                                III                                      II                                      I                      ]  
and that SMSPrime represents the 3xc3x979 transformation in TABLE 1, and RDower1 represents the 9xc3x973 transformation in TABLE 2, the derived lead set D(dI, dII, dIII, dV1, dV2, dV3, dV4, dV5, and dV6) can be computed as follows:
D=RDowerxc2x7SMSPrimexc2x7L
The MVWS 3 is located at a remote location, such as at a nurses station, and receives the as input signals the output signals put on the Communication Network 2 from the Bedside Monitor 1. A software application within MVWS 3 consumes the set of derived leads available on the Network 2 and reconstructs the X, Y, Z leads using the Edenbrandt transformation represented in TABLE 3 of FIG. 2.
Given that N is an 8xc3x971 lead array representing derived lead values at a particular instant,   N  =      [                            dV1                                      dV2                                      dV3                                      dV4                                      dV5                                      dV6                                      dII                                      dI                      ]  
and that Edenbrandt represents the 3xc3x978 transformation in TABLE 3, the derived Frank lead set F(X, Y, Z) can be computed as follows:
F=Edenbrandtxc2x7N
The same algebraic formulas which were used to derive the augmented leads on Monitor 1 is used to derive the augmented leads locally on the MVWS 3. These are actually derived augmented leads, since the input to the equations are in fact derived leads dI and dII.
Clinicians have expressed a preference for sampled (i.e., actual) limb leads over the derived limb leads in such an ECG application. Unfortunately, the prior work does not provide for the use of sampled limb leads. If sampled limb leads are substituted for the derived dI and dII, the reconstruction of the Frank X, Y, Z lead using the Edenbrandt transformation outlined in TABLE 3 fails.
Accordingly, there remains a need for an improved method and apparatus for developing Frank x, y and z signals for analyzing activity of the human heart, and which uses a reduced number of derived signals. The present invention fulfills these needs and provides other related advantages. More specifically, the present invention reduces bandwidth requirements in the signal transmission network, as well as the number of CPU cycles required to reconstruct the Frank Lead(s) X, Y, and Z (signals dI and dII are no longer needed, as well as the calculations according for these signals, compare Table 3 to Table 6). It also allows the actually sampled limb leads to be maintained throughout the system, rather than derived limb leads.
In an ECG monitoring and analyzing system of the type where electrodes are placed on a subject for detecting electrical activity of a heart, and where the electrode placement is such that Frank Leads X, Y and Z can be constructed from the detected electrical activity, a method and apparatus for ECG signal transformation to Frank X, Y and Z leads, comprises an input, responsive to a set of input signals corresponding to no more than derived chest leads dV1, dV2, dV3, dV4, dV5 and dV6, a memory for storing coefficients of a transformation matrix, and an output, for providing transformation matrix output signals corresponding to application of said transformation matrix coefficients to said input signals, said output signals corresponding to said Frank X, Y and Z leads.
The invention reduces bandwidth requirements in a ECG signal communication network, as well as the complexity of the processing required for constructing the Frank Leads. Other features and advantages of the present invention will also become apparent from the following more detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of a preferred example, the principles of the invention.